1. Field of the Invention
The present invention relates to a wide-angle lens system to be employed in a single lens reflex camera, and to a focusing method of the wide-angle lens system.
2. Description of the Prior Art
In a single reflex camera, a photographing lens system thereof needs to secure the back focal distance of a predetermined length, since a quick-return mirror is provided between the photographing lens system and the image-forming plane. Accordingly, in a wide-angle lens system, a retrofocus lens system in which the back focal distance is longer than the focal length has generally been employed. In such a retrofocus-type wide-angle lens system, particularly, in a wide-angle lens system in which all the lens groups are advanced upon focusing, image-forming performance at closer object distances becomes worse, because of large fluctuations in aberrations.
It is an object of the present invention to provide a wide-angle lens system which has (i) no substantial distance-dependent changes inaberrations, in particular, at closer distances, (ii) suitable image-forming performance with respect to an object at infinity toward an object at a closer distance, (iii) a large aperture (about F1.8 to F2.0), and (iv) a half-angle-of-view of about 35xc2x0; and to provide a focusing method for the wide-angle lens system.
In order to achieve the above-mentioned object, there is provided a wide-angle lens system including a negative first lens group and a positive second lens group, in this order from the object. The second lens group includes a positive 2F sub-lens group (front sub-lens group) and a positive 2R sub-lens group (rear sub-lens group), in this order from the object. Upon focusing on an object at infinity toward an object at a closer distance (i.e., focusing from an infinite object distance to a closer object distance), an Fa lens group (first focusing lens group) constituted by the first lens group and the positive 2F sub-lens group (front sub-lens group) which are arranged to move integrally, and an Fb lens group (second focusing lens group) constituted by the positive 2R sub-lens group (rear sub-lens group) are independently moved from the image side toward the object side so that the distance between the Fa lens group (first focusing lens group) and the Fb lens group (second focusing lens group) are made shorter. Furthermore, the wide-angle lens system satisfies the following conditions:
0.5 less than H1/H2R less than 0.9xe2x80x83xe2x80x83(1) 
1.7 less than nPAVxe2x80x83xe2x80x83(2) 
0.5 less than Xan/Xbn less than 1.0xe2x80x83xe2x80x83(3) 
wherein
H1 designates the radius of a bundle of axial light rays on the most object-side surface of the first lens group, when an object at an infinite photographing distance is in an in-focus state;
H2R designates the radius of a bundle of axial light rays on the most object-side surface of the positive 2R sub-lens group (rear sub-lens group) of the second lens group, when an object at an infinite photographing distance is in an in-focus state;
nPAV designates the averaged refractive indexes of the d-line with respect to positive lens elements in the positive second lens group;
Xan designates a distance along which the Fa lens group (first focusing lens group) is advanced, the distance of which is defined by the position of the Fa lens group (first focusing lens group) corresponding to an infinite photographing distance and the position thereof corresponding to the shortest photographing distance; and
Xbn designates a distance along which the Fb lens group (second focusing lens group) is advanced, the distance of which is defined by the position of the Fb lens group (second focusing lens group) corresponding to an infinite photographing distance and the position thereof corresponding to the shortest photographing distance.
According to a focusing method for the wide-angle lens system, (i) the wide-angle lens system is constituted by a retrofocus-type lens system including a negative first lens group and a positive second lens group, in this order from the object, (ii) the positive second lens group includes a positive 2F sub-lens group (front sub-lens group) and a positive 2R sub-lens group (rear sub-lens group), in this order from the object, and (iii) the wide-angle lens system satisfies the above conditions (1) and (2); and the method includes the following steps:
providing an Fa lens group (first focusing lens group) constituted by the first lens group and the positive 2F sub-lens group (front sub-lens group) so that these lens groups are integrally moved;
providing an Fb lens group (second focusing lens group) constituted by the positive 2R sub-lens group (rear sub-lens group); and
in order to satisfy the above condition (3) upon focusing on an object at infinity toward an object at a closer distance, moving the Fa lens group (first focusing lens group) and the Fb lens group (second focusing lens group) independently from the image side toward the object side so that the distance between the Fa lens group (first focusing lens group) and the Fb lens groups (second focusing lens group) are made shorter.
Condition (3) specifies the ratio of the traveling distance of the Fa lens group (first focusing lens group) to that of the Fb lens group (second focusing lens group) with respect to an in-focus state of an object at infinity and an in-focus sate of an object at the shortest photographing distance. Between the above two in-focus states at infinity and the shortest photographing distance, focusing operation can be carried out either by varying the ratio of the traveling distance of the Fa lens group (first focusing lens group) to that of the Fb lens group (second focusing lens group) when an in-focus state at infinity is changed to an in-focus state of an object at an arbitrary distance, or by setting a substantially constant ratio thereof. The former is known as a non-linear focusing method, and the latter is known as a linear focusing method. In the case of the non-linear focusing method, the Fa lens group (first focusing lens group) and the Fb lens group (second focusing lens group) preferably satisfy the following conditions:
0.5 less than Xan/Xbn less than xcex94Xai/xcex94Xbi less than 1.0xe2x80x83xe2x80x83(4) 
xe2x88x920.1 less than xcex2 less than 0xe2x80x83xe2x80x83(5) 
wherein
xcex94Xai designates the slope of a tangent at a point, corresponding to an infinite photographing distance, on the moving path of the Fa lens group (first focusing lens group);
xcex94Xbi designates the slope of a tangent at a point, corresponding to an infinite photographing distance, on the moving path of the moving path of the Fb lens group (second focusing lens group);
X1=xcex1X2+xcex2X22;
X1 designates a distance along which the Fa lens group (first focusing lens group) is advanced, the distance of which corresponds to a photographing distance between the infinite photographing distance and the shortest photographing distance;
X2 designates a distance along which the Fb lens group (second focusing lens group) is advanced, the distance of which corresponds to a photographing distance between an infinite photographing distance and the shortest photographing distance;
xcex1 designates a first coefficient; and
xcex2 designates a second coefficient.
In the wide-angle lens system according to the present invention, at lease one aspherical surface is preferably provided in the second lens group. More preferably, the aspherical surface is provided in the positive 2R sub-lens group (rear sub-lens group), and satisfies the following condition:
0.2 less than xcex94V less than 0xe2x80x83xe2x80x83(6) 
wherein
xcex94V designates an aspherical-surface term in an aberration coefficient.
The wide-angle lens system of the present invention can satisfy the following condition:
|f/fFa| less than 0.25xe2x80x83xe2x80x83(7) 
wherein
f designates the focal length of the entire lens system; and
fFa designates the focal length of the Fa lens group (first focusing lens group).
The present disclosure relates to subject matter contained in Japanese Patent Applications No. 2000-265256 (filed on Sep. 1, 2000) which is expressly incorporated herein in its entirety.